The present invention relates to data storage devices, and in particular to an enhanced method and apparatus for detecting defects in a data storage device.
Modern computer systems process and store ever increasing volumes of data. Data is commonly stored on rotating magnetic disk drives. A rotating magnetic disk drive typically contains one or more disks rigidly attached to a spindle which is driven at a constant rotational velocity. Data is magnetically encoded on concentric tracks on the surfaces of the disks. In order to read or write data, one or more transducer heads, each attached to a movable actuator arm, are positioned over respective concentric tracks of data. Electrical circuits in the drive read signals generated from the transducer as the moving disk passes along it, or write data to the disk surface by energizing the transducer as the disk moves.
The rotating magnetic disk drive is a complex electro-mechanical assembly, requiring a high degree of sophisticated technology in its design and manufacture. In order to store data at the high densities demanded by the marketplace, all moving parts must be fabricated to exacting tolerances. This is particularly true of the disk surface itself. In addition, even where all component parts are produced within tolerances, minor errors or variations in any number of assembly tasks can introduce defects in the finished disk drive product.
Although great care may be taken during all stages of disk drive manufacture to assure quality, it is inevitable that, due to the inherent complexity and close tolerances of the device, a proportion of the finished disk drive assemblies will not function as they should. To avoid shipping defective disk drives to a customer, it is necessary to inspect and test each individual disk drive assembly.
One source of potential defects is in the disk surface itself. Proper disk operation requires that the transducer head maintain a very close proximity to the surface of the disk as the disk rotates. This distance between transducer head and disk surface is known as the xe2x80x9cflyheightxe2x80x9d. At the same time, physical contact between the transducer head and the disk surface is to be avoided, because it can damage the head. As data storage densities have increased and the size of the transducer has correspondingly been reduced, it has become necessary to reduce the desired flyheight of the head. This in turn requires that the disk surface be extremely even. A microscopic outcropping in the disk surface or particle resting on the disk surface may collide with the head, causing device failure.
If a disk surface irregularity is sufficiently large, a single collision or a small number of collisions produced during testing may cause the head to fail, and thus be detected by conventional means. In most cases, however, the surface irregularity must collide with the head many times before the head is damaged. A disk drive with such a surface irregularity may easily pass a functional test, yet fail when exposed to repeated head collisions in actual use.
In order to verify the uniformity of the disk surface, it has been known to test the disk and spindle assembly by mounting the disk and spindle assembly in a special test bed and inserting a specially constructed head/actuator test assembly from the test bed in the disk. This head/actuator test assembly contains small piezoelectric sensors which can sense sudden acceleration of a head produced by a collision. There are several problems with this approach to testing the disk surface. The test introduces additional handling because each disk and spindle assembly must be physically secured to a test bed apparatus, and the special heads inserted into the disk and spindle assembly. The test bed apparatus itself is expensive. Both the test bed apparatus and the disk drives under test are very delicate and subject to damage. Finally, the nature of the test demands that the disk drive protective enclosure be installed after the test, and thus it is possible for contaminant particles to settle on the surface of the disk or other defects to be introduced after it has passed the test and before the protective enclosure is installed.
An alternative method of testing is to place piezoelectric sensors on the head/actuator assemblies of the disk drive that are ultimately shipped to customers. With this alternative, it is possible to test the disk drive after the protective enclosure is installed, thus avoiding some of the handling problems associated with a special test bed. However, this alternative requires piezoelectric sensors in the finished product, considerably increasing the cost of the product.
IBM Technical Disclosure Bulletin, Vol. 32, No. 9A, p. 280 (February 1990) discloses another alternative testing method. In this method, a harmonic ratio flyheight (HRF) signal is derived from the data read signal produced by the transducer. This alternative presents advantages over the use of piezoelectric sensors, in that defects can be detected using the actual transducer hardware in place, without the insertion of a test head/actuator or the addition or piezoelectric sensors to the finished product. However, the HRF signal is a less accurate indicator of defects, and is in particular susceptible of giving xe2x80x9cfalse positivesxe2x80x9d.
It is, therefore, an object of the present invention to provide an enhanced data storage device.
Another object of this invention is to provide an enhanced method and apparatus for manufacturing data storage devices.
Another object of this invention is to provide an enhanced method and apparatus for detecting defects in a data storage device.
Another object of this invention is to increase the accuracy of detecting defects in a data storage device.
Another object of this invention is to reduce the frequency of undetected defects in a data storage device.
Another object of this invention is to reduce the frequency of rejecting data storage devices during inspection and testing, when the device is in fact not defective.
Another object of this invention is to increase the reliability of data storage devices.
Another object of this invention is to reduce the cost of manufacturing and testing data storage devices.
In a preferred embodiment, a disk drive assembly is tested during the manufacturing process. Testing is performed after the mechanical disk assembly, comprising the spindle, motor, disks, actuator, and transducer heads, is completely assembled and enclosed in its protective enclosure. A known data pattern is written to selected tracks on the disk surface, and the data is read back. During the read process, the analog read signal is sampled at first and third harmonic rates, and the logarithmic ratio of the two sampled signals used to derive a harmonic ratio flyheight (HRF) signal. In the absence of certain abnormalities, the HRF signal approximates the distance between the surface of the disk and the transducer head (flyheight).
When a transducer head passes over an outcropping or particle on the disk surface of sufficient size, a collision occurs, causing the transducer to be lifted momentarily above its normal flyheight. Thus, when the amplitude of the HRF signal exceeds a predetermined clipping level, a possible disk defect is indicated.
Not all abnormalities which may cause the HRF signal to exceed the clipping level are in fact caused by outcroppings or contamination. Certain abnormalities, such as magnetic voids, may have no effect on long term reliability of the disk drive assembly and can either be ignored or circumvented through conventional disk formatting techniques. Different types of abnormalities exhibit different HRF signal profiles.
In order to characterize the possible defect, a window of the HRF signal samples in the vicinity of the suspected abnormality is digitized and used as the input to a neural network. The neural network is trained with actual HRF samples from previously detected disk drive abnormalities, which have been categorized by microscopic examination or other means. Thus, the neural network will recognize and distinguish different types of abnormalities. The neural network produces an output for the HRF signal samples from the drive being tested which indicates whether the defect is of a type which can be ignored, or of a type which requires scrapping or rework of the drive.
In a preferred embodiment, the neural network is an artificial neural network executing on a suitably programmed general purpose computer.